Recently efforts have been made to develop apparatus and methods for depositing materials over relatively large areas, such amorphous semiconductor alloys that can be doped to form p-type and n-type materials. These alloys can be used to produce p-n, p-i-n and other electronic device structures that are useful in photovoltaic and other applications.
Examples of such semiconductor alloy deposition efforts are described in U.S. Pat. Nos. 4,226,898 to Stanford R. Ovshinsky and Arun Madan, and 4,400,409, and 4,410,558, both to Masatsugu Izu, Vincent D. Cannella and Stanford R. Ovshinsky. These patents, incorporated herein by reference, disclose batch and continuous glow discharge deposition of amorphous semiconducting alloys containing compensating agents to reduce the concentration of localized states to a level acceptable for use in electronic devices. The deposited alloys may also contain bandgap adjusting elements and dopants.
As explained in the referenced patents to Izu et al., p-n and p-i-n structures can be deposited by passing a continuous substrate through a plurality of mutually isolated deposition chambers. Some of the chambers may contain selected dopant sources in addition to deposition sources so that doped and undoped layers may be sequentially deposited to prepare a desired electronic device structure. An important feature of these inventions is the ability to fabricate electronic devices continuously on a flexible substrate web that moves through the chambers during the deposition process. Such a flexible substrate may be paid out from one coil and taken up into another coil and may be hundreds of meters long and tens of centimeters wide. Therefore, a very large area of semiconducting material may be deposited in one passage of such a flexible substrate web through the deposition chambers.
The speed with which a web substrate may pass through a deposition chamber is limited, in one instance, by the rate at which material is deposited and the thickness of the layer required to be deposited in a particular chamber. As disclosed in the referenced patents, the glow discharge plasma in a deposition chamber may be initiated and sustained by radio frequency energy. It is known that glow discharge deposition rates may be increased by increasing the frequency of the energy supplied to the plasma to the microwave range, i.e. about 1000 MHz and up, corresponding to wavelengths shorter than about 0.3 meters. For an example, see U.S. Pat. No. 4,401,054 to Matsuo et al. The use of microwave energy also increases the efficiency of the deposition process, reduces the electron temperature in the glow discharge plasma and thereby reduces ion bombardment damage to the substrate and previously deposited layers of materials on the substrate.
However, use of microwave energy for plasma deposition over a large area, such as the width of the substrate web described above, creates a number of problems. For example, the wavelength of the microwave energy is of the same order of the magnitude as the web width so that standing waves may exist in a deposition chamber. These standing waves can produce non-uniform thicknesses in deposited layers across the width of a substrate. Therefore, use of microwave energy for glow deposition of materials has generally been confined to laboratory scale, small area depositions.